Have you been doing alot of offroad?

You could have hit stuff that scratches it and let's the rust on it when you don't wash it off or dirty water gets on it and dries.

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1979 Ford F150 4X4, 7.5L, C6, 38s Super Swampers TSL

1978 Ford Courier 4x4, 2.3L carbed, 5 spd manual, dry-rotted tires

1985 Ford Ranger 4x4, 2.3L FI, 5 spd manual, 29s Power Kings

1990 Ford Bronco 4x4, 4.9L, 5 spd manual, 31s Maxxis Bighorns